A technique is known in which a NOx storage reduction catalyst (hereinafter, also referred to as an NSR catalyst) is arranged in an exhaust passage of an internal combustion engine. The NSR catalyst stores NOx in exhaust gas when oxygen concentration of exhaust gas flowing in is high and reduces stored NOx when oxygen concentration of exhaust gas flowing in is low and a reducing agent is present. In addition, a three-way catalyst can also be equipped with functions similar to an NSR catalyst.
Furthermore, a selective catalytic reduction NOx catalyst (hereinafter, also referred to as a SCR catalyst) can be provided on a downstream side of a three-way catalyst or an NSR catalyst. The SCR catalyst is a catalyst that selectively reduces NOx using a reducing agent. At the three-way catalyst or the NSR catalyst, H2 is generated from CO or HC in exhaust gas and, in turn, NH3 is generated from the H2 and NOx. The NH3 is used as a reducing agent at the SCR catalyst. When combustion is performed in the internal combustion engine at a lower air-fuel ratio (rich air-fuel ratio) than a stoichiometric air-fuel ratio, CO or HC can be discharged from the internal combustion engine. If NOx is stored by the three-way catalyst or the NSR catalyst, by operating the internal combustion engine at a rich air-fuel ratio, NH3 generated at the three-way catalyst or the NSR catalyst can be supplied to the SCR catalyst as a reducing agent.
There are known techniques in which, when a reforming catalyst having H2-generating capability is provided on an upstream side of a NSR catalyst and an air-fuel ratio is set to a rich air-fuel ratio in order to reduce NOx stored in the NSR catalyst, the air-fuel ratio is controlled to mainly cause a partial oxidation reaction if a temperature of the reforming catalyst is equal to or lower than a prescribed value, and the air-fuel ratio is controlled to mainly cause a steam reforming reaction or an aqueous gas shift reaction if the temperature of the reforming catalyst is higher than the prescribed value (for example, refer to Patent Document 1).
However, when generating H2 by a steam reforming reaction or an aqueous gas shift reaction, a gas component that can effectively generate H2 differs depending on catalyst temperature. Simply controlling the air-fuel ratio does not guarantee an optimum gas component for generating H2. Therefore, there is room for improvement in increasing efficiency when generating H2 and NH3.